The 6c
Good! A three piece razor is a good place to start, plus I’m 99% sure that it uses standard threads. If you like the handle then there are many vendors that sell head-only versions of razors for a decent price difference.
The main reason I asked is that I have their basic TTO(twist to open) called the Rookie and I don’t like it at all. The design is so wide it makes it impossible to shave under the nose.
Of the blades you mentioned, one of my recent surprises was the Derby Premium. It is a medium sharpness and is quite comfortable to my skin. I had avoided Derby products since I started because of the bad press it got, so I was pleasantly surprised that it agreed with me.